An ROADM system is widely used in wavelength division multiplexing (WDM) optical communication. In the ROADM system, branching and coupling of wavelength-multiplexed optical signals are performed by the use of an optical branching/coupling device or the like.
FIG. 10 is a diagram showing a configuration of a general ROADM system 7000. In the ROADM system 7000, an optical branching unit 1000 branches an optical signal received from an upstream transmission line, and an optical demultiplexing unit 4000 demultiplexes the branching optical signal. As a result, optical transmission/reception devices 6000-1 to 6000-n each receives only an optical signal of required wavelength. Optical signals transmitted from respectively the optical transmission/reception devices 6000-1 to 6000-n are multiplexed together at an optical multiplexing unit 5000, and the multiplexed optical signal is transmitted to an optical coupling unit 3000. A wavelength blocker 2000 eliminates, from an optical signal inputted to wavelength blocker 2000 from the optical branching unit 1000, optical signal components of the same wavelengths as those for optical signals transmitted or received by the optical transmission/reception devices 6000-1 to 6000-n, and outputs the resultant optical signal to the optical coupling unit 3000. The optical coupling unit 3000 multiplexes the optical signal from the wavelength blocker 2000 with the optical signal from the optical multiplexing unit 5000, and outputs the multiplexed optical signal.
In recent years, as a result of Color-less Direction-less Contention-less (CDC)-ROADM technology being put into practical use, application of CDC to existing networks has been advanced. A CDC-ROADM system is an ROADM system capable of performing add/drop of optical signals without generating any contention between the optical signals, even when they have any wavelengths or any routes. The CDC function is realized by solving the dependency of a wavelength multiplexing/demultiplexing unit on route and wavelength.
A configuration of a general CDC-ROADM system is described in Patent Literature 1 (PTL 1). The CDC-ROADM system described in PTL 1 realizes the CDC function by the use of a plurality of Wavelength Selective Switches (WSSs), optical couplers and cross-connect switches.